For a wind turbine arranged to generate electric power to an electric grid, power efficiency is a crucial parameter. To provide a high power efficiency the wind turbine must be connected to the electric grid with as few shut down events as possible.
One known source of error causing shut down of the wind turbine is the position sensing or position encoding of the rotor of the power generator. E.g., doubly fed induction type generators require precise sensing of angular position of the generator rotor, e.g., within +/−1°, in order for the control systems to match the electric power signals from the generator to the electric grid for reliable and smooth delivering of electric power to the electric grid. Even short term errors or no position signal can result in emergency shut down of the generator system due to high currents, thereby resulting in poor overall power efficiency of the wind turbine. Further, even minor errors in the position signal can cause unintended voltage peaks from the generator system thus reducing quality in the electric power delivered to the electric grid.
Position sensing devices, often denoted position encoders, are known to eventually produce errors in their output position signals. Further, such devices are often mechanically connected to the shaft of the generator and therefore includes parts running at the speed of the generator shaft, e.g., 1500 rpm or more. Consequently, bearings etc. of the position encoders have a limited lifetime, and therefore early detection of problems with the position encoder is vital with respect to avoid complete damage of the position encoder, which will shut down the wind turbine until the position encoder is replaced.
It is also possible to calculate an angular position of the rotor based solely on the electric output signals from the generator and the electric grid to which the generator delivers power. This eliminates the need for a mechanical position encoder. However, such calculations are complex and require a large amount of processing power. Further, if the voltage is low on the electric grid, then the calculated values are unreliable, and at zero voltage on the electric grid calculation is not possible.